Alkanes comprise a significant fraction of the world's petroleum and natural gas resources and have the potential to be a useful source of carbon for large-scale synthesis. Methane, the smallest alkane and the principal component of natural gas, is an abundant and inexpensive natural resource. Despite these attributes, it is typically only used as a fuel for power generation. The reason for methane's under utilization is that there are few commercially viable methods for converting methane to a product that is chemically-useful due to the strength of its covalent carbon-hydrogen (C—H) bonds, which are among the strongest of all hydrocarbons. The search for catalysts that can facilitate C—H bond activation in methane and other low molecular-weight alkanes is an area of research with considerable industrial significance.
The most common chemical use of methane is to convert it (by an indirect oxidation process) to methanol, a commercially important alcohol that is one of the top 25 chemicals produced worldwide. The conversion is generally carried out at high temperatures and pressures in a two-stage steam reforming process to form synthesis gas (carbon monoxide and hydrogen), and is coupled with a methanol synthesis process that dates back to the 1920s. The process is expensive, energy intensive, and impractical for use in the remote locations where many of the methane reserves are found. As a result, the direct oxidative conversion of methane to an easily transportable liquid such as methanol has been extensively investigated for decades.
Although various parties have been seeking simpler, more efficient methods for converting methane to methanol, no such methods have been commercialized due to the difficulty in finding a sufficiently active and selective catalyst. Although low temperature selective methane oxidation by transition metal complexes in solution has been the focus of substantial effort since the 1970s, the most promising catalysts so far described by Periana et al. 1993 and 1998 (Periana R. A., et al. (1993) Science 259:340, Periana R. A. et al., (1998) Science 280:560-564) have not yet been commercialized. The process described by Periana et al. in 1998 utilizes a platinum bipyrimidine catalyst and requires concentrated sulfuric acid. This catalyst converts the methane to methyl bisulfate (CH3OSO3H), which can then be converted to methanol.
Ionic liquids are salts consisting of ions that exist in the liquid state at ambient temperatures, or salts that have melting points below around 300° C. Ionic liquids typically consist of organic nitrogen-containing heterocyclic cations and inorganic anions. Ionic liquids offer numerous advantages over conventional organic solvents for carrying out organic reactions, including very low vapor pressure, lack of flammability, and the capacity to be functionalized to suit particular reactions. Unlike conventional molten salts (for example, molten sodium chloride), ionic liquids often melt below 300° C. Since the melting points are low, ionic liquids can act as solvents in which reactions can be performed, and because the liquid is made of ions rather than molecules, such reactions often provide distinct selectivities and reactivities as compared to conventional organic solvents. In addition, their non-volatility results in low impact on the environment and human health, and they are recognized as solvents for “green” chemistry.
Ionic liquids have been disclosed for use as solvents for a broad spectrum of chemical processes. These ionic liquids, which in some cases can serve as both catalyst and solvent, are attracting increasing interest from industry because they promise significant environmental benefits. Several patent applications, including international PCT publication Nos. WO 95/21871, WO 95/21872, and WO 95/21806 relate to ionic liquids and their use to catalyze hydrocarbon conversion reactions such as polymerization and alkylation reactions.
There is a significant need in the art for readily available hydrocarbon sources that require a less expensive plant, which is cheaper to run, uses less energy, and produces fewer pollutants than the current technology. Ionic liquids may provide a new approach for facilitating this difficult yet important chemical reaction.